1. Field of the Invention
The present invention relates to an evaporator of the type comprising a heat-exchanger body which has main passages placed in a heat-exchange relationship, means for forming a pool of the liquid to be evaporated so that it flows through at least a first of said main passages, and means for introducing a refrigerant into at least a second of said main passages so that evaporation of the liquid is ensured.
The invention applies, for example, to a reboiler-condenser for a double-column air distillation plant.
In such a reboiler-condenser, oxygen-rich liquid coming from the bottom of the low-pressure column is evaporated in the reboiler-condenser by condensing a nitrogen-rich gas withdrawn from the top of the medium-pressure column.
More generally, an air separation apparatus such as a double distillation column comprises several types of heat exchanger.
2. Description of the Prior Art
A main heat exchanger serves to cool the feed air for the apparatus to the distillation temperature by heat exchange with one or more fluids coming from the distillation apparatus. In some cases, these are pressurized liquids from the apparatus which vaporize against the air to be distilled in the exchanger. These exchangers are normally made entirely of aluminum or copper or alloys of these metals (WO 95/28610).
For safety reasons, these liquids sometimes vaporize in a dedicated exchanger, or evaporator, against a single fluid such as air or nitrogen.
The apparatus also comprises at least one reboiler-condenser which is a heat exchanger placed inside or outside the column. These reboiler-condensers are usually made entirely of copper, stainless steel, nickel or aluminum and consist of at least two circuits which are connected to the rest of the plant by means of pipes welded to the equipment.
The exchangers used in air separation apparatuses comprise heat exchanger bodies which are often produced in the form of parallel aluminum plates having a similar contour and being brazed together.
In general in exchangers which serve as evaporators, an oxygen-rich liquid is vaporized as it flows a countercurrent with respect to a nitrogen-rich gas (such as air or nitrogen with a purity greater than 80%).
In order to improve the performance of these evaporators, it is possible to use evaporators called xe2x80x9ctrickling-film or falling-film evaporatorsxe2x80x9d, that is to say of the aforementioned type, and in which the oxygen-rich liquid of the pool is delivered at the top of the evaporator in the form of a very thin film which flows vertically through the first main passages and part of which is vaporized by heat exchange with the passages dedicated to the nitrogen-rich gas in a cocurrent manner.
EP-0,795,349 describes the case in which such an evaporator is combined with a thermosiphon-type evaporator (a so-called pool evaporator, that is to say an evaporator completely immersed in the liquid where the recirculation of the oxygen-rich liquid takes place by virtue of the hydraulic thrust due to the difference in density between the pool and the liquid being vaporized in the passages).
In the brazed-plate exchanger bodies used in evaporators of the aforementioned falling-film type, such as that in EP-A-0,130,122, the liquid is distributed between many passages consisting of vertical corrugations inserted between two sheets called separating sheets and thus constituting heat fins and, because of the pitch of these corrugations, the brazed-plate heat exchanger bodies have exchange surfaces of very large area.
Therefore, when the entire surface is wetted, the liquid film will be very thin and to avoid dry evaporation in the bottom of the first main passages or should there be a distribution defect, excess liquid is made to flow through the heat exchanger body. This excess liquid generally requires liquid to be recycled by means of a pump.
In evaporators of the aforementioned type, called pool evaporators, recirculation of the liquid is also maintained in order to prevent dry evaporation at the top of the first main passages.
Moreover, U.S. Pat. No. 5,699,671 describes an evaporator having a tube-type exchanger body, arranged vertically, in which the gaseous nitrogen condenses in contact with its tubes.
It has been found, especially in falling-film reboiler-condensers, that solid contaminants such as, for example, hydrocarbons or nitrogen protoxide, may accumulate in the passages dedicated to the oxygen-rich fluid, and this can lead to the blockage of these passages.
Such a blockage therefore degrades the operation of the reboiler-condenser.
One object of the invention is to solve this problem by providing an evaporator of the aforementioned type which reduces the risk of blockage of the passage or passages dedicated to the liquid to be evaporated.
Another object of the invention is to minimize the recirculation of the liquid to be evaporated in evaporators of the aforementioned type and to ensure operating safety and optimum performance.
For this purpose, the subject of the invention is an evaporator of the aforementioned type, wherein the or each first main passage possesses, in the free section transverse to the direction of flow of the liquid to be evaporated, at least one continuous free-flow region sufficiently extended to allow the liquid to get round a deposit of impurities, or, when the main passages are bounded by vertical plates having a substantially similar contour and being parallel to one another and spaced apart so as to form the flat main passages, at least a first main passage is either narrower than the second main passage and contains neither an exchange corrugation nor an auxiliary passage, or contains one or more closed auxiliary passages which extend over most of the length of the heat-exchanger body parallel to the direction of flow of the liquid to be evaporated, the walls of the auxiliary passage(s) touching the plates defining the main passage.
Preferably, all the first main passages contain at least one closed auxiliary passage.
Thus, the liquid sent into the auxiliary passage passes through the evaporator without contacting the plates defining the first main passages. As far as possible, the liquid must be prevented from flowing between the outside of the auxiliary passage and the passages defined by the plates.
One means of avoiding this problem consists in forming the auxiliary passages in a block of material (for example made of aluminum, nickel or copper). If the block has substantially dimensions of a first main passage, the liquid cannot flow outside the auxiliary passages, which are all cylindrical holes passing through the block.
Ideally, the maximum width of an auxiliary passage is greater than 50% of the distance between two adjacent plates.
In order to prevent the accumulation of impurities, the internal surface of the auxiliary passage or of each auxiliary passage comprises only curved surfaces and, possibly, convexities. The absence of cavities in the passages of the first set (xe2x80x9cliquidxe2x80x9d passages) has never been proposed in the prior art.
According to one embodiment, at least one, and preferably all, of the first main passages contain several auxiliary passages consisting of a series of cylindrical tubes parallel to one another and each having a diameter at least equal to 50% of the separation between two adjacent plates.
According to another embodiment, at least one and preferably all of the first main passages contain several auxiliary passages consisting of tubes, each having an internal surface with at least three identical convexities and curved surfaces connecting the convexities.
The adjacent tubes may or may not be contiguous.
Preferably, there are means for directing liquid into the or each auxiliary passage and/or liquid distribution means consisting of predistribution openings, these openings allowing this liquid to fall over a packing located above the means for directing liquid into one or each auxiliary passage.
In one embodiment, the means for directing the liquid into the passages are inclined tips, the ends of which lie above the inside of the auxiliary passage (or of an auxiliary passage).
The evaporator may be a main exchanger which serves to cool purified air to its distillation temperature, a subcooler or the reboiler-condenser of a double column.
The subject of the invention is also an air distillation plant comprising at least one evaporator as defined above.